In the manufacture of integrated circuits of this kind, a substrate of a semiconductor material such as silicon of one conductivity type (e.g. P) is formed, usually by epitaxial growth, with an overlying layer of the opposite conductivity type (N) subdivided, by barrier zones of the first conductivity type (P), into several regions wherein the various circuit elements such as transistors, diodes, capacitors and resistors can be formed by the selective introduction of impurities through suitable masks of silicon oxide, for example. Thus, an intermediate or insular zone of the first conductivity type (P) within such an isolated region can serve as the base of an NPN transistor (or PNP transistor if the conductivity types were interchanged) whose emitter is constituted by a section of the second conductivity type, preferably of higher impurity concentration (N+), embedded in that zone. The collector of the transistor, represented by the surrounding region of the same conductivity type (N), may include a low-resistance zone with a higher impurity concentration (N+) similar to that of the emitter.
Such an isolated zone of the same conductivity type as the substrate, but separated therefrom by a part of the overlying epitaxial layer, can also be used as a portion of a diode whose remainder is constituted by the isolated region of this layer in which that zone is received. A diode of this description may have a relatively high reverse-voltage threshold e.g. of about 55 V, suitable for use in a power stage of an amplifier also including a transistor as described above. A problem arising in these instances, however, resides in the fact that the diode forms with the substrate a parasitic transistor (in the assumed instance of PNP type) which tends to generate an appreciable leakage current when the diode is biased for forward conduction. This is true because, as a rule, the junction between the substrate and the epitaxial layer is reverse-biased to minimize conduction thereacross and to reduce the effect of stray capacitances existing between that junction and ground.
Various solutions have already been proposed for limiting that leakage current by restricting the flow of charge carriers from the diode toward the substrate. One such solution consists in creating, prior to the formation of the epitaxial layer, a low-resistance stratum of the same conductivity type (N+) buried between that layer and the substrate, generally within the confines of the barrier surrounding the isolated region. According to another earlier proposal, the low-resistance zone of like conductivity type (N+) included in the isolated region is extended into contact with the buried stratum so as virtually to enclose, from a distance, the isolated zone of the other conductivity type (P) by a shell capable of neutralizing the charge carriers (holes) injected into the isolated region by the forwardly biased intermediate zone. Even with these precautions, however, the aforementioned parasitic transistor can still drain significant amounts of charge carriers from the forwardly conducting diode, especially in the case of linear integrated power circuits.